Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	$ 1	$ 15	$ 2	$ 30
Non-service (benefit) cost	(35)	(22)	(69)	(43)
Net periodic (benefit) cost	(34)	(7)	(67)	(13)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0	0	0
Interest cost	0	1	1	2
Expected return on plan assets	(1)	(1)	(2)	(2)
Defined Benefit Plan, Amortization of Gain (Loss)	0	0	0	0
Amortization of prior service cost	(20)	(20)	(40)	(40)
Settlements	0	0	0	0
Non-service (benefit) cost	(21)	(20)	(41)	(40)
Net periodic (benefit) cost	(21)	(20)	(41)	(40)
Losses and loss expenses | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	1	0	3
Non-service (benefit) cost	(3)	(2)	(6)	(4)
Losses and loss expenses | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0	0	0
Non-service (benefit) cost	(2)	(2)	(4)	(4)
Administrative Expense | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	1	14	2	27
Non-service (benefit) cost	(32)	(20)	(63)	(39)
Administrative Expense | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0	0	0
Non-service (benefit) cost	(19)	(18)	(37)	(36)
Foreign Plan [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	1	2	2	5
Interest cost	5	7	11	14
Expected return on plan assets	(11)	(11)	(21)	(22)
Defined Benefit Plan, Amortization of Gain (Loss)	1	1	1	1
Amortization of prior service cost	0	0	0	0
Settlements	0	0	0	0
Non-service (benefit) cost	(5)	(3)	(9)	(7)
Net periodic (benefit) cost	(4)	(1)	(7)	(2)
UNITED STATES | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	13	0	25
Interest cost	25	29	50	59
Expected return on plan assets	(56)	(48)	(112)	(95)
Defined Benefit Plan, Amortization of Gain (Loss)	0	0	0	0
Amortization of prior service cost	0	0	0	0
Settlements	1	0	2	0
Non-service (benefit) cost	(30)	(19)	(60)	(36)
Net periodic (benefit) cost	$ (30)	$ (6)	$ (60)	$ (11)